This invention is related to an injection molder and particularly to an injection molder which enables to perform high speed molding and precise molding. 2. Description of the Prior Art:
In a conventional injection molder, a screw for injection is rotatably and forward and backward movably arranged within a heating cylinder. The rotation of the screw causes synthetic resin in a molten condition to be fed in an accumulating chamber located at the forward end of a heating cylinder, and thereafter, the synthetic resin is injected in a cavity to fill the cavity therewith through forward movement of the screw.
In a conventional injection molding method as described above, after the molten synthetic resin has been charged, injection takes place, and thereby it has a disadvantage in which it takes time and is a barrier to high-speed molding.
FIG. 22 is an explanatory drawing indicating an actual cycle of a whole injection molding process. In a diagram, A illustrates a mold opening process, B the ejecting process of an eject member, C the return process of the eject member, D a mold closing process, E an injection process, and F a charge process. As shown in this diagram, in a conventional injection molding method, each process is carried out serially in terms of time, and thereby the actual cycle time is increased and this also causes impediment of improvement of the operating speed of a molder.
Recently, for the reason that feedback control of a position is effected easily, response is excellent, and motion is stable even at a high speed, utilization of a servo motor as a driving source for an injection molder is studied in every way. A control method, as published in, for example, the patent gazette publication No. 58-179631, is proposed as a control method for the back pressure of a screw in a servo motor-driven injection molder.
In the back pressure control method, the back pressure of the screw is controlled in a way that forward movement of the screw, occurring when molten synthetic resin is charged through rotation of the screw, is converted into rotary movement by means of a gear means, the rotation force is adjusted by a brake means, and this causes control of the back pressure of the screw.
This method, however, has a disadvantage in which, in order to convert backward movement of the screw into rotary movement, a gear means is employed, or a brake means is used to control rotary movement produced resulting from conversion, and thereby making stable and accurate control of the back pressure difficult. Meanwhile, conventionally, a toggle type mold clamping device and a direct pressure type mold clamping device are employed as the clamping device of an injection molder. Such devices are both driven hydraulically, and a clamping operation is effected so that the flow direction of pressure oil therein is switched to a forward or a reverse direction at a given time. Thereby, a valve is required for switching motion, a lag occurs until an oil pressure is increased to a given pressure after the flow of pressure oil is switched, and this causes impediment of high-speed molding.